Coulometric detection method is often used to measure remaining battery power when electric vehicles are moving. The coulometric detection method is called Ampere-hour method or integration method, which can be used when the vehicles are moving, as can be seen in FIG. 5. When the battery is charged, the battery power is accumulated from an initial point based on the ampere-hour method to obtain the battery power at any time during charging. Likewise, when the battery is discharged, the battery power decreases from based on the ampere-hour method, and the battery power can be represented by I=∫T0i(t)dt. According to the concept of energy conservation, the coulometric detection method has a certain degree of precision, and an accumulated number using the Ampere-hour method during charging should be equal to a decreasing number using the Ampere-hour method during discharging. However, accumulating errors may occur in the coulometric detection method after every charge/discharge process due to the error in the manufacturing process of electronic components, measurement analysis, sampling, etc. The accumulating error increases with the usage of the battery over time, so the remaining battery power shown in the vehicle does not reflect real reaming battery power and a user has no way to know the real remaining battery power, which may causes inconvenience to the user. This is the problem the present invention would like to solve.